1. Field of the Invention
The present invention relates to a driving force control apparatus for a four-wheel drive vehicle which controls the distribution of a driving force to main driving wheels and sub-driving wheels.
2. Description of the Prior Art
A conventional driving force control apparatus for a four-wheel drive vehicle of the type mentioned above is known, for example, from one disclosed in Japanese Patent Publication No. 8-19981. In this four-wheel drive vehicle (hereinafter simply called the “vehicle”), a torque of a power plant having an engine, a transmission and the like is transmitted to front wheels and rear wheels from an output shaft of the power plant through a front wheel and a rear wheel propeller shaft, respectively. Also, a hydraulic clutch is disposed between the output shaft and the rear wheel propeller shaft, such that the driving force control apparatus controls a fastening force of this clutch to control the a torque transmitted to the rear wheels, i.e., the distribution of torques transmitted to the front wheels and rear wheels.
In the foregoing driving force control apparatus, a speed difference sensor detects a difference in rotational speed between the front and rear propeller shafts, and a target value for a torque to be transmitted to the rear wheels is set in accordance with the detected difference in rotational speed. The control apparatus comprises a control unit which stores three types of control characteristics (dry, wet, snow), different from one another, for setting the target value, such that one of the control characteristics is selected in response to a driver's operation on a selector switch. The control apparatus sets a target value for a torque to the rear wheels in accordance with the difference in rotational speed based on the selected control characteristics, and controls a fastening force of the clutch to control a torque transmitted to the rear wheel to the set target value. In this event, when the difference in rotational speed is below a first predetermined value or above a second predetermined value larger than the first predetermined value for a predetermined time period or longer due to variations in surface resistance, the selected control characteristic is changed to another control characteristic to set a different target value for the torque transmitted to the rear wheels. In this way, the torque transmitted to the rear wheels can be increased or decreased to transmit a proper torque to the rear wheels in accordance with the varying surface resistance, thereby properly distributing the torque to the front wheels and rear wheels.
However, the driving force control apparatus disclosed in Japanese Patent Publication No. 8-19981 described above can experience difficulties in stably launching a vehicle for the reasons set forth below on a road which is not uniform in surface condition and has variations in surface resistance for each wheel (hereinafter called the “split road”).
When the control characteristic is set to be suitable for a level road, a vehicle can be smoothly launched on a level road, for example, even if the surface resistance is smaller on the front wheel side so that the front wheels are slipping, by increasing a torque distributed to the rear wheels from the difference in rotational speed between the front and rear wheels. However, if the front wheels are slipping when the vehicle is launched on a gradient road, the control characteristic suitable for a level load can distribute an insufficient torque to the rear wheels to possibly fail a stable launch.
Conversely, when the control characteristic is set to be suitable for a gradient road, though the vehicle can be smoothly launched on a gradient road, the vehicle cannot be stably launched on a level road due to an excessive torque distributed to the rear wheels, when the front wheels are slipping, to cause the vehicle to slide sideways. Also, in the event of a launch associated with slipping as mentioned above, even if the selected control characteristic is changed to another control characteristic to distribute a different torque to the rear wheels in response, the vehicle cannot still be stably launched due to a difference in torque which occurs upon switching the control characteristic.